A porous honeycomb structure constituted by cell partition walls (ribs) forming a plurality of cells adjacent to each other (a cell group) and an outer wall surrounding and holding the outermost cells present at the periphery of the cell group, is in wide use as a filter (a diesel particulate filter, DPF) for capturing and removing the particulate substance contained in a dust-containing fluid (e.g. an exhaust gas from diesel engine) or as a catalyst carrier for loading thereon a catalyst component which purifies the harmful substance contained in an exhaust gas; and refractory silicon carbide (SiC) is being used as a matrix material of the porous honeycomb structure.
As such a honeycomb structure, there is known, for example, a porous silicon carbide-based catalyst carrier of honeycomb structure which is obtained by using, as a starting material, an impurity-containing silicon carbide powder having a given specific surface area, forming the starting material into a given shape, drying the formed body, and firing the dried formed body in a temperature range of 1,600 to 2,200° C. (see, for example, JP-A-1994-182228).
In production of the above-mentioned porous silicon carbide-based catalyst carrier, the silicon carbide component vaporizes from the surfaces of the silicon carbide powder (particles) and condenses at the contacting areas (neck areas) of the particles; thereby, the neck areas grow and a bonded state is obtained in the sintering (necking) by recrystallization of the silicon carbide powder per se. However, the vaporization of silicon carbide requires a very high firing temperature, inviting a high cost, and the high-temperature firing of the material having high thermal expansion coefficient causes a reduction in firing yield, which have been a problem.
Further, when it is attempted to produce, by the sintering by recrystallization of the silicon carbide powder per se, a filter of high porosity, particularly a filter having a porosity of 50% or more, there has been a problem in that the sintering mechanism does not function sufficiently, thereby the growth of neck area is inhibited, consequently the filter produced has a low strength.
As a conventional technique for solving these problems, there were disclosed a porous honeycomb structure containing refractory particles as an aggregate, particularly silicon carbide and metallic silicon and a process for producing the porous honeycomb structure (see, for example, JP-A-2002-201082). However, this porous honeycomb structure has had a problem in that the structure, when loaded with, for example, an alkali metal (e.g. potassium, K) as an NOx occlusion catalyst or the like, shows a rapid deactivation of catalyst. This is a problem which tends to occur when a porous body containing silicon carbide as the main component is used as a catalyst carrier, regardless of the shape of the carrier.